MpInitLib: Only allocate below 1MB memory for 16bit code

MpInitLib: Only allocate below 1MB memory for 16bit code

Today's implementation allocates below 1MB memory for the 16bit, 32bit
and 64bit code.

But it's not necessary since now the 32bit and 64bit code run at high
memory no matter in PEI and DXE phase.

The patch simplifies the logic to remove the code that handles the
case when WakeupBufferHigh is 0.
It also reduce the memory foot print under 1MB by allocating
memory for 16bit code only.

MP_CPU_EXCHANGE_INFO is still under 1MB which is immediate
after the 16bit code.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>